(i) Field of the Invention
This invention relates to a fiber reinforced resin molded article for electromagnetic waves. More particularly, this invention relates to a fiber reinforced resin (hereinafter sometimes referred to as FRP) molded article which contains an electroconductive nonwoven fabric-resin composite layer having a flat or curved surface and provides the properties of uniformly reflecting and shielding VHF or shorter electromagnetic waves, and to a method for production thereof. Some molded articles of the present invention are especially useful as antennas for receiving or transmitting electromagnetic waves in the range of VHF to EHF.
(ii) Description of the Prior Art
Hitherto, for providing resin molded articles with electromagnetic wave-reflecting properties (hereinafter referred to as EMWR properties) or electromagnetic wave-shielding properties (hereinafter referred to as EMWS properties or EMI shielding properties), there have been conducted coating of substrate articles with electroconductive paints; application of metallic materials on the substrate articles by means of flame spraying, chemical plating, vapor deposition, sputtering, or ion-plating; addition of electroconductive fillers, metal foils, metal fibers, metal ribbons or metal flakes to the molding resins; and the like. The application of these materials to thermo-setting resin articles, however, had some troubles with respect to performances, stability, costs and/or processability of the resulting products. For example, coating with electroconductive paints caused oxidative deterioration, formation of cracking and peeling, etc. The flame spraying of metals required a large apparatus with high cost, needed a pre-treatment step, and also caused toxic metal vapors. In the chemical plating, the plastics to be plated are restricted to ABS resin and some other similar resins, and also a costly apparatus is required.
In the case of a parabola antenna for reflecting electromagnetic waves, such antennas of metal (normally of aluminum) have been fabricated carefully with high cost and can hardly be repaired when the reflective surface thereof was once damaged.
In the case of a parabola antenna made of FRP, the reflective surface thereof having uniformly electroconductive surface is required in order to obtain necessary performances. In this connection, it has been considered that an electroconductive nonwoven fabric can not be used successfully for the above mentioned purpose in a compression molding method, because the fibers of the nonwoven fabrics are moved or broken by the molding pressure and ununiform flow of a resin component, etc. and an ununiform reflective surface is produced. Thus, an FRP plate having the EMWR or EMWS properties, wherein a sheet molding compound containing a large amount of an electroconductive short fiber is used instead of the nonwoven fabric, has been produced by compression molding. The performance of this FRP plate containing such short fiber and no nonwoven fabric, however, is unstable because the electroconductive short fiber is dispersed ununiformly throughout the FRP body by the compression molding step to give an ununiform reflective surface. See, for example, CONDUCTIVE POLYMER (1981), pages 49-55, Plenum Publishing Corporation, New York. Even when an extremely large amount of the short fiber and resin materials are used regardless of high costs, the resulting FRP plate contains a very thick, ununiformly dispersed electroconductive layer which may fail to exhibit uniformly reflecting properties.
A hand lay-up molding method has problems in both precise surface characteristics and productivity. An injection molding method suffers from precise surface characteristics of the resulting product. Such inferior surface characteristics as waviness on the surface caused by shrinkage upon hardening of resin materials have adverse effects on reflective loss of electromagnetic waves and reflecting properties as well as on antenna performances.